1. Field of the Invention
This invention relates to absorbent material comprising specific modified polyvinyl alcohol type polymers and to absorbent articles comprising the same. More particularly, it relates to absorbent materials, which absorb aqueous fluids rapidly and swell uniformly, and which comprise modified polyvinyl alcohol type polymers obtained by reacting in anhydrous conditions polyvinyl alcohol type polymers with cyclic acid anhydrides and causing partial crosslinking of said polymers as well as to absorbent articles comprising said absorbent materials and fluid absorbing holders therefor, such as diapers, sanitary napkins, sanitary tampons, blood absorbents for use in surgical operations, bandages and dressings. (Hereinafter, the abbreviation "PVA" denotes "polyvinyl alcohol".)
2. Description of the Prior Art
In recent years, with the increase of the range of use of hydrophilic polymeric materials in the fields of medical industry, food industry and agriculture, water-insoluble but hydrophilic or water absorbing polymeric materials, among others, have found use as materials for separation or purification purposes and carriers for liquid chromatorgraphy, culture mediums for microbes and plants, and medical materials such as contact lenses and dressings for surgical sutures, and other various uses taking advantages of their water absorbing or holding power. Among these applications, those making the most use of water absorbing power, include the uses in the sanitary field such as diapers, sanitary napkins, sanitary tampons, blood absorbents for use in surgical operations, bandages and dressings. These uses require polymeric materials not only capable of rapidly absorbing or removing large quantities of body fluids such as urine and blood but also capable of giving to the structure of the products made therefrom a property such that the body fluids absorbed by the structure neither wet the skin nor give an unpleasant feeling.
As polymeric materials to be used for these applications, there have been proposed several materials derived from natural and synthetic polymeric materials, such as modified polyethylene oxides, crosslinked polyvinyl pyrrolidones, crosslinked sulfonated polystyrenes and saponified starch-acrylonitrile graft copolymers.
However, their water absorbing power, except for the case of the saponified starch-acrylonitrile copolymer, is weak, rendering them unsatifactory as water absorbing materials. In the case of the saponified starch-acrylonitrile graft copolymer which can absorb water in amounts more than 30 times its own weight, too, there are several problems from a practical standpoint, for example, the decay of the starch constituent may possibly lead to destruction of the gel structure. The conventional absorbent materials, such as those from pulps, which do not contain such synthetic or semi-synthetic absorbent materials as mentioned above, have disadvantages such as that, owing to their low absorbing power per unit of volume, increased volumes are required to increase absorption, raising the cost of production and making the products uncomfortable to wear.